An encoder is a motion detector that provides closed-loop feedback to a motor control system. A typical optical encoder design includes an emitter/detector module, which can be designed in either a transmissive, reflective, or imaging configuration. When operated in conjunction with either a codewheel or codestrip, the encoder translates rotary motion or linear motion, as appropriate, into a two or three-channel digital output.
FIG. 1A depicts a transmissive-type encoder. In this transmissive configuration, the encoder includes a light source 108 which transmits light through a codewheel or codestrip 104 and the light which passes through the codewheel or codestrip 104 is detected at a light detector 112.
FIG. 1B depicts a reflective-type encoder. In this reflective configuration, the light source 108 transmits light toward a codewheel or codestrip 104 and the light which reflects off the codewheel or codestrip 104 is detected at the light detector 112.
FIG. 1C depicts an imaging-type encoder. In this imaging configuration, the light source 108 illuminates a codewheel or codestrip 104 and the light detector 112 takes a series of images of the illuminated codewheel or codestrip 104 to detect motion of the codewheel or codestrip 104.
In reflective encoders, a lens may be provided over the light source 104 to focus the light onto the codewheel or codestrip 104. Light is either reflected or not reflected back to the lens over the light detector 112. As the codewheel or codestrip 104 moves, an alternating pattern of light and dark corresponding to the pattern of the bars and spaces falls upon the light detector 112. Often, the light detector 112 includes an array of photodiodes and these photodiodes detect these interruptions (light and dark) and the outputs of the photodiodes are processed by a signal processor of the encoder to produce digital waveforms. These encoder outputs can be used to provide information about position, velocity, and acceleration of the motor.
Reflective encoders provide advantages of compact size and easy assembly as compared to transmissive and imaging-type encoders. Particularly, the light source 108 and light detector 112 are provided on the same substrate, thereby allowing low product profile after assembly, fewer parts, and fewer assembly processes. However, reflective encoders suffer from its low image contrast that restricts the encoder from operating at high speed and resolution. Stray light reflected from the internal lens surfaces reach the photodetector arrays (PDAs), and this stray reflected light contributes to electrical noise in the reflective encoder.
Current solutions for dealing with this noise problem include incorporating a light baffle component in the encoder. Specifically, the light baffle is provided between the light source 108 and light detector 112 to block stray light from reaching the light detector 112. The light baffle helps to reduce the noise experienced by reflective encoders. Additional details of encoders which attempt to reduce the noise experienced at the light source 112 are described in one or more of U.S. Pat. Nos. 7,182,258; 7,304,294; and 7,795,576, each of which are hereby incorporated herein by reference in their entirety.
It would be desirable to provide an encoder, specifically a reflective encoder, that also addresses the stray light/noise issues of prior art reflective encoders without requiring the additional light baffle component. Specifically, incorporation of a light baffle component into the encoder requires special-purpose machinery, which is quite expensive, and adds additional steps to the manufacturing process, which adds time and expense to the manufacturing process.